Air conditioning systems with a re-heating system for active humidity control can overcool the air provided to the conditioned space while performing active dehumidification. This occurs because the re-heat coil is not sized to provide neutral supply air temperature. For example, condenser re-heat systems, like the Lennox Humiditrol® EDA, still have a Sensible cooling-to-Total cooling ratio (S/T) of about 0.25 so that some sensible cooling is occurring while dehumidification is being required by the humidistat setting.
A typical control scheme uses “cooling priority” first to satisfy the cooling requirement from the thermostat, and then, if there is excess humidity detected by the humidity sensor, the vapor compression circuit and evaporator continue to cool the air so that the excess humidity can be removed. Since, under most conditions, there is a positive S/T ratio, the space continues to be cooled during this continued dehumidification mode. This results in overcooling of the air and conditioned space. Some thermostats even employ an “overcooling limit” to stop the dehumidification mode from lowering the conditioned air too far below the temperature setpoint even if the desired relative humidity has not been met. The fact that enhanced dehumidification is only enabled after the temperature setpoint has been achieved means that more than the minimum amount of energy is being used to provide the space with conditioned air.
Accordingly, what is needed in the art is an air conditioning system that avoids the wasted energy of overcooling the air in order to achieve the desired relative humidity.